Method and apparatus for transmitting and receiving frame supporting short MAC header in wireless LAN system

ABSTRACT

The present invention relates to a wireless communication system and, more particularly, to a method and an apparatus for transmitting and receiving a frame supporting a short MAC header in a wireless LAN system. A method for receiving a frame by a station (STA) in a wireless communication system in accordance with one embodiment of the present invention may comprise the steps of: receiving the frame including a sequence control (SC) field; determining a packet number (PN) using the value of the SC field and a partial PN value stored in the STA; and performing decryption for the frame using the PN. When the decryption is performed after reordering a block ACK for the frame, an operation can be performed to increase the partial PN value stored in the STA by 1 if the sequence number value of the SC field of the received frame is less than the previous sequence number value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2014/006004, filed on Jul. 4, 2014,which claims the benefit of U.S. Provisional Application No. 61/926,940,filed on Jan. 13, 2014, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting andreceiving a frame supporting a short Medium Access Control (MAC) headerin a Wireless Local Area Network (WLAN) system.

BACKGROUND ART

Various wireless communication technologies systems have been developedwith rapid development of information communication technologies. WLANtechnology from among wireless communication technologies allowswireless Internet access at home or in enterprises or at a specificservice provision region using mobile terminals, such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), etc. on the basis of Radio Frequency (RF) technology.

In order to obviate limited communication speed, one of the advantagesof WLAN, the recent technical standard has proposed an evolved systemcapable of increasing the speed and reliability of a network whilesimultaneously extending a coverage region of a wireless network. Forexample, IEEE 802.11n enables a data processing speed to support amaximum High Throughput (HT) of 540 Mbps. In addition, Multiple Inputand Multiple Output (MIMO) technology has recently been applied to botha transmitter and a receiver so as to minimize transmission errors aswell as to optimize a data transfer rate.

DISCLOSURE Technical Problem

Machine-to-Machine (MTM) communication is under discussion as afuture-generation communication technology. A technical standardsupporting M2M communication is also being developed as Institute ofElectrical and Electronics Engineers (IEEE) 802.11ah in IEEE 802.11WLAN. For M2M communication, a scenario in which a very small amount ofdata is transmitted and received at a low rate from time to time in anenvironment with a huge number of devices may be considered.

An object of the present invention devised to solve the conventionalproblem is to provide a method for managing a Sequence Number (SN) inthe case of a short Medium Access Control (MAC) header, in order to savethe power of a Station (STA) and prevent malfunction of the STA. Also,another object of the present invention is to provide a method forconfiguring an encrypted data unit in the case of a short MAC header.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned herein willbe apparent from the following description to one of ordinary skill inthe art to which the present invention pertains.

Technical Solution

In an aspect of the present invention, a method for receiving a frame ata Station (STA) in a wireless communication system includes receiving aframe including a Sequence Control (SC) field, determining a PacketNumber (PN) using a value of the SC field and a partial PN value storedin the STA, and performing decryption for the frame using the PN. If thedecryption for the frame is performed after block Acknowledgment (ACK)reordering, an operation is performed, the operation increasing thepartial PN value stored in the STA by 1 when a sequence number of the SCfield of the received frame is smaller than a previous sequence number.

In another aspect of the present invention, a STA for receiving a framein a wireless communication system includes a transceiver and aprocessor. The processor is configured to receive a frame including anSC field, to determine a PN using a value of the SC field and a partialPN value stored in the STA, and to perform decryption for the frameusing the PN. If the decryption for the frame is performed after blockAcknowledgment (ACK) reordering, an operation is performed, theoperation increasing the partial PN value stored in the STA by 1 when asequence number of the SC field of the received frame is smaller than aprevious sequence number.

In the above aspects of the present invention, the followings areapplicable.

If a block ACK is not used for the MPDU and the sequence number of theSC field of the received frame is smaller than the previous sequencenumber, the partial PN value stored in the STA may be increased by 1.

The block ACK reordering may include ordering a plurality of framesincluding the frame in an ascending order of sequence numbers.

The PN may be 48 bits long and determined by concatenating PN0, PN1,PN2, PN3, PN4, and PN5, each being 8 bits long.

The SC field may be set to a value obtained by concatenating PN0 andPN1.

The partial PN value may be obtained by concatenating PN2, PN3, PN4, andPN5.

When the sequence number is rolled over, the sequence number of the SCfield of the received frame may be smaller than the previous sequencenumber.

The frame may be a MAC Protocol Data Unit (MPDU).

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the present invention, a method and apparatus for managinga Sequence Number (SN) in the case of a short Medium Access Control(MAC) header can be provided. Also, a method and apparatus forconfiguring an encrypted data unit in the case of a short MAC header canbe provided.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows an Institute of Electrical and ElectronicEngineers (IEEE) 802.11 system according to one embodiment of thepresent invention.

FIG. 2 exemplarily shows an IEEE 802.11 system according to anotherembodiment of the present invention.

FIG. 3 exemplarily shows an IEEE 802.11 system according to stillanother embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating a Wireless Local AreaNetwork (WLAN) system.

FIG. 5 is a flowchart illustrating a link setup process for use in theWLAN system.

FIG. 6 is a conceptual diagram illustrating a backoff process.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

FIG. 9 is a conceptual diagram illustrating a power managementoperation.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof a Station (STA) having received a Traffic Indication Map (TIM).

FIG. 13 is a conceptual diagram illustrating a group-based AssociationIdentifier (AID).

FIG. 14 is a conceptual diagram illustrating a frame structure for usein IEEE 802.11.

FIG. 15 is a conceptual diagram illustrating an example of a long-rangePhysical Layer Convergence Protocol (PLCP) frame format.

FIG. 16 is a conceptual diagram illustrating a repetition method forconstructing a PLCP frame format of a 1 MHz bandwidth.

FIG. 17 is a conceptual diagram illustrating an example of an extendedcapability element according to an embodiment.

FIG. 18 is a block diagram illustrating Counter mode with Cipher-blockchaining Message authentication code Protocol (CCMP) encapsulation.

FIG. 19 is a conceptual diagram illustrating a frame control field of ashort MAC header according to an embodiment.

FIG. 20 is a conceptual diagram illustrating an example of AdditionalAuthentication Data (AAD) according to an embodiment.

FIG. 21 is a conceptual diagram illustrating a Nonce according to anembodiment.

FIG. 22 is a conceptual diagram illustrating an exemplary encrypted MACProtocol Data Unit (MPDU) according to an embodiment.

FIG. 23 is a diagram illustrating a MAC Service Data Unit (MSDU)reception flow in a MAC data plane architecture.

FIG. 24 is a flowchart illustrating a method according to an embodiment.

FIG. 25 is a block diagram of a wireless apparatus according to anembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, Code DivisionMultiple Access (CDMA), Frequency Division Multiple Access (FDMA), TimeDivision Multiple Access (TDMA), Orthogonal Frequency Division MultipleAccess (OFDMA), Single Carrier Frequency Division Multiple Access(SC-FDMA), and the like. CDMA may be embodied through wireless (orradio) technology such as Universal Terrestrial Radio Access (UTRA) orCDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as Global System for Mobile communication (GSM)/General PacketRadio Service)/EDGE (Enhanced Data Rates for GSM Evolution (GPRS). OFDMAmay be embodied through wireless (or radio) technology such as Instituteof Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, and Evolved UTRA (E-UTRA). For clarity, thefollowing description focuses on IEEE 802.11 systems. However, technicalfeatures of the present invention are not limited thereto.

WLAN System Structure

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent STA mobility for a higherlayer may be provided by mutual operations of the components. A BasicService Set (BSS) may correspond to a basic constituent block in an IEEE802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are shown and two STAsare included in each of the BSSs (i.e. STA1 and STA2 are included inBSS1 and STA3 and STA4 are included in BSS2). An ellipse indicating theBSS in FIG. 1 may be understood as a coverage area in which STAsincluded in the corresponding BSS maintain communication. This area maybe referred to as a Basic Service Area (BSA). If a STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an Independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand in which other components are omitted, may correspond to a typicalexample of the IBSS. Such configuration is possible when STAs candirectly communicate with each other. Such a type of LAN is notprescheduled and may be configured when the LAN is necessary. This maybe referred to as an ad-hoc network.

Memberships of a STA in the BSS may be dynamically changed when the STAis switched on or off or the STA enters or leaves the BSS region. TheSTA may use a synchronization process to join the BSS. To access allservices of a BSS infrastructure, the STA should be associated with theBSS. Such association may be dynamically configured and may include useof a Distribution System Service (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a Distribution System (DS), a Distribution SystemMedium (DSM), and an Access Point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by Physicallayer (PHY) performance. In some cases, such restriction of the distancemay be sufficient for communication. However, in other cases,communication between STAs over a long distance may be necessary. The DSmay be configured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a Wireless Medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data may move between theBSS and the DS through the AP. For example, STA2 and STA3 shown in FIG.2 have STA functionality and provide a function of causing associatedSTAs (STA1 and STA4) to access the DS. Moreover, since all APscorrespond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not always beidentical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to a STAaddress of the AP may always be received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an ExtendedService Set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a Logical Link Control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One or more IBSSs orESS networks may be physically located in the same space as one or moreESS networks. This may correspond to an ESS network form in the case inwhich an ad-hoc network operates in a location in which an ESS networkis present, the case in which IEEE 802.11 networks of differentorganizations physically overlap, or the case in which two or moredifferent access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, a STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as laptop computers or mobile phones,handled directly by users. In FIG. 4, STA1, STA3, and STA4 correspond tothe non-AP STAs and STA2 and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a Wireless Transmit/Receive Unit (WTRU), a User Equipment(UE), a Mobile Station (MS), a mobile terminal, or a Mobile SubscriberStation (MSS). The AP is a concept corresponding to a Base Station (BS),a Node-B, an evolved Node-B (e-NB), a Base Transceiver System (BTS), ora femto BS in other wireless communication fields.

Layer Architecture

An operation of a STA in a WLAN system may be described from theperspective of a layer architecture. A processor may implement the layerarchitecture in terms of device configuration. The STA may have aplurality of layers. For example, the 802.11 standards mainly deal witha MAC sublayer and a PHY layer on a Data Link Layer (DLL). The PHY layermay include a Physical Layer Convergence Protocol (PLCP) entity, aPhysical Medium Dependent (PMD) entity, and the like. Each of the MACsublayer and the PHY layer conceptually includes management entitiescalled MAC sublayer Management Entity (MLME) and Physical LayerManagement Entity (PLME). These entities provide layer managementservice interfaces through which a layer management function isexecuted.

To provide a correct MAC operation, a Station Management Entity (SME)resides in each STA. The SME is a layer independent entity which may beperceived as being present in a separate management plane or as beingoff to the side. While specific functions of the SME are not describedin detail herein, the SME may be responsible for collectinglayer-dependent states from various Layer Management Entities (LMEs) andsetting layer-specific parameters to similar values. The SME may executethese functions and implement a standard management protocol on behalfof general system management entities.

The above-described entities interact with one another in variousmanners. For example, the entities may interact with one another byexchanging GET/SET primitives between them. A primitive refers to a setof elements or parameters related to a specific purpose. AnXX-GET.request primitive is used to request a predetermined MIBattribute value (management information-based attribute information). AnXX-GET.confirm primitive is used to return an appropriate MIB attributeinformation value when the Status field indicates “Success” and toreturn an error indication in the Status field when the Status fielddoes not indicate “Success”. An XX-SET.request primitive is used torequest setting of an indicated MIB attribute to a predetermined value.When the MIB attribute indicates a specific operation, the MIB attributerequests the specific operation to be performed. An XX-SET.confirmprimitive is used to confirm that the indicated MIB attribute has beenset to a requested value when the Status field indicates “Success” andto return an error condition in the Status field when the Status fielddoes not indicate “Success”. When the MIB attribute indicates a specificoperation, it confirms that the operation has been performed.

Also, the MLME and the SME may exchange various MLME GET/SET primitivesthrough an MLME Service Access Point (MLME_SAP), in addition, variousPLME_GET/SET primitives may be exchanged between the PLME and the SMEthrough a PLME_SAP, and exchanged between the MIME and the PLME throughan MLME-PLME_SAP.

Link Setup Process

FIG. 5 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow a STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

Link setup process is described referring to FIG. 5.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 5 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of the active scanning, a STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which Access Point(AP) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be a STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. A STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives the beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning, theactive scanning is more advantageous than the passive scanning in termsof delay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process in such a manner that theauthentication process can be clearly distinguished from the securitysetup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a Quality of Service (QoS) map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on anExtensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

WLAN Evolution

In order to obviate limitations in WLAN communication speed, IEEE802.11n has recently been established as a communication standard. IEEE802.11n aims to increase network speed and reliability as well as toextend a coverage region of the wireless network. In more detail, IEEE802.11n supports a High Throughput (HT) of a maximum of 540 Mbps, and isbased on MIMO technology in which multiple antennas are mounted to eachof a transmitter and a receiver.

With the widespread use of WLAN technology and diversification of WLANapplications, there is a need to develop a new WLAN system capable ofsupporting a HT higher than a data processing speed supported by IEEE802.11n. The next generation WLAN system for supporting Very HighThroughput (VHT) is the next version (for example, IEEE 802.11ac) of theIEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systemsrecently proposed to support a data process speed of 1 Gbps or more at aMedium Access Control Service Access Point (MAC SAP).

In order to efficiently utilize a radio frequency (RF) channel, the nextgeneration WLAN system supports Multi User Multiple Input MultipleOutput (MU-MIMO) transmission in which a plurality of STAs cansimultaneously access a channel. In accordance with the MU-MIMOtransmission scheme, the AP may simultaneously transmit packets to atleast one MIMO-paired STA.

In addition, a technology for supporting WLAN system operations inwhitespace has recently been discussed. For example, a technology forintroducing the WLAN system in whitespace (TV WS) such as an idlefrequency band (for example, 54˜698 MHz band) left because of thetransition to digital TV has been discussed under the IEEE 802.11 ofstandard. However, the above-mentioned information is disclosed forillustrative purposes only, and the whitespace may be a licensed bandcapable of being primarily used only by a licensed user. The licenseduser may be a user who has authority to use the licensed band, and mayalso be referred to as a licensed device, a primary user, an incumbentuser, or the like.

For example, an AP and/or STA operating in the White Space (WS) mustprovide a function for protecting the licensed user. For example,assuming that the licensed user such as a microphone has already used aspecific WS channel acting as a divided frequency band on regulation ina manner that a specific bandwidth is occupied from the WS band, the APand/or STA cannot use the frequency band corresponding to thecorresponding WS channel so as to protect the licensed user. Inaddition, the AP and/or STA must stop using the corresponding frequencyband under the condition that the licensed user uses a frequency bandused for transmission and/or reception of a current frame.

Therefore, the AP and/or STA must determine whether to use a specificfrequency band of the WS band. In other words, the AP and/or STA mustdetermine the presence or absence of an incumbent user or a licenseduser in the frequency band. The scheme for determining the presence orabsence of the incumbent user in a specific frequency band is referredto as a spectrum sensing scheme. An energy detection scheme, a signaturedetection scheme and the like may be used as the spectrum sensingmechanism. The AP and/or STA may determine that the frequency band isbeing used by an incumbent user if the intensity of a received signalexceeds a predetermined value, or when a DTV preamble is detected.

Machine-to-Machine (M2M) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in theIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines, or may also be referred to asMachine Type Communication (MTC) or M2M communication. In this case, themachine may be an entity that does not require direct handling andintervention of a user. For example, not only a meter or vending machineincluding a RF module, but also a user equipment (UE) (such as asmartphone) capable of performing communication by automaticallyaccessing the network without user intervention/handling may be anexample of such machines. M2M communication may include Device-to-Device(D2D) communication and communication between a device and anapplication server, etc. As exemplary communication between the deviceand the application server, communication between a vending machine andan application server, communication between the Point Of Sale (POS)device and the application server, and communication between an electricmeter, a gas meter or a water meter and the application server.M2M-based communication applications may include security,transportation, healthcare, etc. In the case of considering theabove-mentioned application examples, M2M communication has to supportthe method for sometimes transmitting/receiving a small amount of dataat low speed under an environment including a large number of devices.

In more detail, M2M communication must support a large number of STAs.Although the current WLAN system assumes that one AP is associated witha maximum of 2007 STAs, various methods for supporting other cases inwhich many more STAs (e.g., about 6000 STAs) are associated with one APhave recently been discussed in M2M communication. In addition, it isexpected that many applications for supporting/requesting a low transferrate are present in M2M communication. In order to smoothly support manySTAs, the WLAN system may recognize the presence or absence of data tobe transmitted to the STA on the basis of a Traffic Indication Map(TIM), and various methods for reducing the bitmap size of the TIM haverecently been discussed. In addition, it is expected that much trafficdata having a very long transmission/reception interval is present inM2M communication. For example, in M2M communication, a very smallamount of data (e.g., electric/gas/water metering) needs to betransmitted at long intervals (for example, every month). In addition,the STA operates according to a command received via downlink (i.e., alink from the AP to the non-AP STA) in M2M communication, such that datais reported through uplink (i.e., a link from the non-AP STA to the AP).M2M communication is mainly focused upon the communication schemeimproved on uplink for transmission of the principal data. Therefore,although the number of STAs associated with one AP increases in the WLANsystem, many developers and companies are conducting intensive researchinto an WLAN system which can efficiently support the case in whichthere are a very small number of STAs, each of which has a data frame tobe received from the AP during one beacon period.

As described above, WLAN technology is rapidly developing, and not onlythe above-mentioned exemplary technologies but also other technologiessuch as a direct link setup, improvement of media streaming throughput,high-speed and/or support of large-scale initial session setup, andsupport of extended bandwidth and operation frequency, are beingintensively developed.

Medium Access Mechanism

In the IEEE 802.11 based WLAN system, a basic access mechanism of MediumAccess Control (MAC) is a Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is referred to as aDistributed Coordination Function (DCF) of IEEE 802.11 MAC, andbasically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start its own transmission, establishes a delaytime (for example, a random backoff period) for medium access, andattempts to start frame transmission after waiting for a predeterminedtime. Through application of a random backoff period, it is expectedthat multiple STAs will attempt to start frame transmission afterwaiting for different times, resulting in minimum collision.

In addition, IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 6 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 6. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count has a value of a PacketNumber (PN), and may be set to one of 0 to CW values. In this case, CWrefers to a Contention Window parameter value. Although an initial valueof the CW parameter is denoted by CWmin, the initial value may bedoubled in case of a transmission failure (for example, in the case inwhich ACK of the transmission frame is not received). If the CWparameter value is denoted by CWmax, CWmax is maintained until datatransmission is successful, and at the same time it is possible toattempt to start data transmission. If data transmission was successful,the CW parameter value is reset to CWmin. Preferably, CW, CWmin, andCWmax are set to 2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymonitors the medium while counting down the backoff slot in response tothe decided backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle state, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, an unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of a failure in datatransmission. In this case, each of STA4 and STA5 increases the CW valuetwo times, and STA4 or STA5 may select a random backoff count value andthen perform countdown. Meanwhile, STA1 waits for a predetermined timewhile the medium is in the occupied state due to transmission of STA4and STA5. In this case, if the medium is in the idle state, STA1 waitsfor the DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For the virtual carriersensing, MAC of the WLAN system can utilize a Network Allocation Vector(NAV). In more detail, by means of the NAV value, the AP and/or STA,each of which currently uses the medium or has authority to use themedium, may inform another AP and/or another STA for the remaining timein which the medium is available. Accordingly, the NAV value maycorrespond to a reserved time in which the medium will be used by the APand/or STA configured to transmit the corresponding frame. A STA havingreceived the NAV value may prohibit medium access (or channel access)during the corresponding reserved time. For example, NAV may be setaccording to the value of a ‘duration’ field of the MAC header of theframe.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be described with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 7(a) exemplarily shows the hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 7(a), STA C may determine that the medium is in the idle state whenperforming carrier sensing before transmitting data to STA B, under thecondition that STA A transmits information to STA B. Since transmissionof STA A (i.e., occupied medium) may not be detected at the location ofSTA C, it is determined that the medium is in the idle state. In thiscase, STA B simultaneously receives information of STA A and informationof STA C, resulting in the occurrence of collision. Here, STA A may beconsidered as a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), under thecondition that STA B transmits data to STA A, STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 8 is a conceptual diagram illustrating Request To Send (RTS) andClear To Send (CTS).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS and CTS. RTS/CTS between two STAs may beoverheard by peripheral STA(s), such that the peripheral STA(s) mayconsider whether information is communicated between the two STAs. Forexample, if STA to be used for data transmission transmits the RTS frameto the STA having received data, the STA having received data transmitsthe CTS frame to peripheral STAs, and may inform the peripheral STAsthat the STA is going to receive data.

FIG. 8(a) exemplarily shows the method for solving problems of thehidden node. In FIG. 8(a), it is assumed that each of STA A and STA C isready to transmit data to STA B. If STA A transmits RTS to STA B, STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 8(b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe actually transmitted can transmit a CTS. STA C receives only the RTSand does not receive the CTS of STA A, such that it can be recognizedthat STA A is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the Reception(Rx) state and the Transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-limited STA (i.e., STAoperated by a battery). Therefore, if STA maintains the Rx standby modeso as to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. In order tosolve the above-mentioned problem, the WLAN system supports a PowerManagement (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning.

The amount of power consumption is reduced in proportion to a specifictime in which the STA stays in the sleep state, such that the STAoperation time is increased in response to the reduced powerconsumption. However, it is impossible to transmit or receive the framein the sleep state, such that the STA cannot mandatorily operate for along period of time. If there is a frame to be transmitted to the AP,the STA operating in the sleep state is switched to the awake state,such that it can transmit/receive the frame in the awake state. On theother hand, if the AP has a frame to be transmitted to the STA, thesleep-state STA is unable to receive the frame and cannot recognize thepresence of a frame to be received. Accordingly, STA may need to switchto the awake state according to a specific period in order to recognizethe presence or absence of a frame to be transmitted to the STA (or inorder to receive a signal indicating the presence of the frame on theassumption that the presence of the frame to be transmitted to the STAis decided).

FIG. 9 is a conceptual diagram illustrating a PM operation.

Referring to FIG. 9, AP 210 transmits a beacon frame to STAs present inthe BSS at intervals of a predetermined time period in steps (S211,S212, S213, S214, S215, S216). The beacon frame includes a TIMinformation element. The TIM information element includes bufferedtraffic regarding STAs associated with the AP 210, and includes specificinformation indicating that a frame is to be transmitted. The TIMinformation element includes a TIM for indicating a unicast frame and aDelivery Traffic Indication Map (DTIM) for indicating a multicast orbroadcast frame.

AP 210 may transmit a DTIM once whenever the beacon frame is transmittedthree times. Each of STA1 220 and STA2 222 is operated in the PS mode.Each of STA1 220 and STA2 222 is switched from the sleep state to theawake state every wakeup interval, such that STA1 220 and STA2 222 maybe configured to receive the TIM information element transmitted by theAP 210. Each STA may calculate a switching start time at which each STAmay start switching to the awake state on the basis of its own localclock. In FIG. 9, it is assumed that a clock of the STA is identical toa clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state in step S221 when AP 210 first transmits the beacon frame instep S211. STA1 220 receives the beacon frame, and obtains the TIMinformation element. If the obtained TIM element indicates the presenceof a frame to be transmitted to STA1 220, STA1 220 may transmit a PowerSave-Poll (PS-Poll) frame, which requests the AP 210 to transmit theframe, to the AP 210 in step S221 a. The AP 210 may transmit the frameto STA1 220 in response to the PS-Poll frame in step S231. STA1 220having received the frame is re-switched to the sleep state, andoperates in the sleep state.

When AP 210 secondly transmits the beacon frame, a busy medium state inwhich the medium is accessed by another device is obtained, the AP 210may not transmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time in step S212. In this case,although STA1 220 is switched to the awake state in response to thebeacon interval, it does not receive the delay-transmitted beacon frameso that it re-enters the sleep state in step S222.

When AP 210 thirdly transmits the beacon frame, the corresponding beaconframe may include a TIM element denoted by DTIM. However, since the busymedium state is given, AP 210 transmits the beacon frame at a delayedtime in step S213. STA1 220 is switched to the awake state in responseto the beacon interval, and may obtain a DTIM through the beacon frametransmitted by the AP 210. It is assumed that DTIM obtained by STA1 220does not have a frame to be transmitted to STA1 220 and there is a framefor another STA. In this case, STA1 220 confirms the absence of a frameto be received in the STA1 220, and re-enters the sleep state, such thatthe STA1 220 may operate in the sleep state. After the AP 210 transmitsthe beacon frame, the AP 210 transmits the frame to the correspondingSTA in step S232.

AP 210 fourthly transmits the beacon frame in step S214. However, it isimpossible for STA1 220 to obtain information regarding the presence ofbuffered traffic associated with the STA1 220 through double receptionof a TIM element, such that the STA1 220 may adjust the wakeup intervalfor receiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by AP 210, the wakeup intervalvalue of the STA1 220 may be adjusted. In this example, STA1 220, thathas been switched to receive a TIM element every beacon interval, may beswitched to another operation state in which STA1 220 can awake from thesleep state once every three beacon intervals. Therefore, when AP 210transmits a fourth beacon frame in step S214 and transmits a fifthbeacon frame in step S215, STA1 220 maintains the sleep state such thatit cannot obtain the corresponding TIM element.

When AP 210 sixthly transmits the beacon frame in step S216, STA1 220 isswitched to the awake state and operates in the awake state, such thatthe STA1 220 is unable to obtain the TIM element contained in the beaconframe in step S224. The TIM element is a DTIM indicating the presence ofa broadcast frame, such that STA1 220 does not transmit the PS-Pollframe to the AP 210 and may receive a broadcast frame transmitted by theAP 210 in step S234. In the meantime, the wakeup interval of STA2 230may be longer than a wakeup interval of STA1 220. Accordingly, STA2 230enters the awake state at a specific time S215 where the AP 210 fifthlytransmits the beacon frame, such that the STA2 230 may receive the TIMelement in step S241. STA2 230 recognizes the presence of a frame to betransmitted to the STA2 230 through the TIM element, and transmits thePS-Poll frame to the AP 210 so as to request frame transmission in stepS241 a. AP 210 may transmit the frame to STA2 230 in response to thePS-Poll frame in step S233.

In order to operate/manage the PS mode shown in FIG. 9, the TIM elementmay include either a TIM indicating the presence or absence of a frameto be transmitted to the STA, or a DTIM indicating the presence orabsence of a broadcast/multicast frame. DTIM may be implemented throughfield setting of the TIM element.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof the STA having received a Traffic Indication Map (TIM).

Referring to FIG. 10, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 10, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be described withreference to FIG. 11.

The STA operations of FIG. 11 in which the STA is switched from thesleep state to the awake state, receives a TIM from the AP, andtransmits the PS-Poll frame to the AP through contention are identicalto those of FIG. 10. If the AP having received the PS-Poll frame doesnot prepare a data frame during the SIFS time, the AP may transmit theACK frame to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of such contending.STA may transmit the ACK frame indicating successful reception of a dataframe to the AP, and may be shifted to the sleep state.

FIG. 12 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM, AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintains the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

TIM Structure

In the operation and management method of the PS mode based on the TIM(or DTIM) protocol shown in FIGS. 9 to 12, STAs may determine thepresence or absence of a data frame to be transmitted for the STAsthrough STA identification information contained in the TIM element. STAidentification information may be specific information associated withan Association Identifier (AID) to be allocated when a STA is associatedwith an AP.

AID is used as a unique ID of each STA within one BSS. For example, AIDfor use in the current WLAN system may be allocated to one of 1 to 2007.In the case of the current WLAN system, 14 bits for AID may be allocatedto a frame transmitted by AP and/or STA. Although the AID value may beassigned a maximum of 16383, the values of 2008˜16383 are set toreserved values.

The TIM element according to legacy definition is inappropriate forapplication of M2M application through which many STAs (for example, atleast 2007 STAs) are associated with one AP. If the conventional TIMstructure is extended without any change, the TIM bitmap sizeexcessively increases, such that it is impossible to support theextended TIM structure using the legacy frame format, and the extendedTIM structure is inappropriate for M2M communication in whichapplication of a low transfer rate is considered. In addition, it isexpected that there are a very small number of STAs each having an Rxdata frame during one beacon period. Therefore, according to exemplaryapplication of the above-mentioned M2M communication, it is expectedthat the TIM bitmap size is increased and most bits are set to zero (0),such that there is needed a technology capable of efficientlycompressing such bitmap.

In the legacy bitmap compression technology, successive values (each ofwhich is set to zero) of 0 are omitted from a head part of bitmap, andthe omitted result may be defined as an offset (or start point) value.However, although STAs each including the buffered frame is small innumber, if there is a high difference between AID values of respectiveSTAs, compression efficiency is not high. For example, assuming that theframe to be transmitted to only a first STA having an AID of 10 and asecond STA having an AID of 2000 is buffered, the length of a compressedbitmap is set to 1990, the remaining parts other than both edge partsare assigned zero (0). If STAs associated with one AP is small innumber, inefficiency of bitmap compression does not cause seriousproblems. However, if the number of STAs associated with one APincreases, such inefficiency may deteriorate overall system throughput.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmittedusing the AIDs. A designated Group ID (GID) is allocated to each group.AIDs allocated on the basis of such group will hereinafter be describedwith reference to FIG. 13.

FIG. 13(a) is a conceptual diagram illustrating a group-based AID. InFIG. 13(a), some bits located at the front part of the AID bitmap may beused to indicate a group ID (GID). For example, it is possible todesignate four GIDs using the first two bits of an AID bitmap. If atotal length of the AID bitmap is denoted by N bits, the first two bits(B1 and B2) may represent a GID of the corresponding AID.

FIG. 13(b) is a conceptual diagram illustrating a group-based AID. InFIG. 13(b), a GID may be allocated according to the position of AID. Inthis case, AIDs having the same GID may be represented by offset andlength values. For example, if GID 1 is denoted by Offset A and LengthB, this means that AIDs (A˜A+B−1) on bitmap are respectively set to GID1. For example, FIG. 13(b) assumes that AIDs (1˜N4) are divided intofour groups. In this case, AIDs contained in GID 1 are denoted by 1˜N1,and the AIDs contained in this group may be represented by Offset 1 andLength N1. AIDs contained in GID 2 may be represented by Offset (N1+1)and Length (N2−N1+1), AIDs contained in GID 3 may be represented byOffset (N2+1) and Length (N3−N2+1), and AIDs contained in GID 4 may berepresented by Offset (N3+1) and Length (N4−N3+1).

In case of using the aforementioned group-based AIDs, channel access isallowed in a different time interval according to individual GIDs, theproblem caused by the insufficient number of TIM elements compared witha large number of STAs can be solved and at the same time data can beefficiently transmitted/received. For example, during a specific timeinterval, channel access is allowed only for STA(s) corresponding to aspecific group, and channel access to the remaining STA(s) may berestricted. A predetermined time interval in which access to onlyspecific STA(s) is allowed may also be referred to as a RestrictedAccess Window (RAW).

Channel access based on GID will hereinafter be described with referenceto FIG. 13(c). If AIDs are divided into three groups, the channel accessmechanism according to the beacon interval is exemplarily shown in FIG.13(c). A first beacon interval (or a first RAW) is a specific intervalin which channel access to a STA corresponding to an AID contained inGID 1 is allowed, and channel access of STAs contained in other GIDs isdisallowed. For implementation of the above-mentioned structure, a TIMelement used only for AIDs corresponding to GID 1 is contained in afirst beacon frame. A TIM element used only for AIDs corresponding toGID 2 is contained in a second beacon frame. Accordingly, only channelaccess to a STA corresponding to the AID contained in GID 2 is allowedduring a second beacon interval (or a second RAW) during a second beaconinterval (or a second RAW). A TIM element used only for AIDs having GID3 is contained in a third beacon frame, such that channel access to aSTA corresponding to the AID contained in GID 3 is allowed using a thirdbeacon interval (or a third RAW). A TIM element used only for AIDs eachhaving GID 1 is contained in a fourth beacon frame, such that channelaccess to a STA corresponding to the AID contained in GID 1 is allowedusing a fourth beacon interval (or a fourth RAW). Thereafter, onlychannel access to a STA corresponding to a specific group indicated bythe TIM contained in the corresponding beacon frame may be allowed ineach of beacon intervals subsequent to the fifth beacon interval (or ineach of RAWs subsequent to the fifth RAW).

Although FIG. 13(c) exemplarily shows that the order of allowed GIDs isperiodical or cyclical according to the beacon interval, the scope orspirit of the present invention is not limited thereto. That is, onlyAID(s) contained in specific GID(s) may be contained in a TIM element,such that channel access to STA(s) corresponding to the specific AID(s)is allowed during a specific time interval (for example, a specificRAW), and channel access to the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks, and channel access toSTA(s) (i.e., STA(s) of a specific group) corresponding to a specificblock having any one of the remaining values other than ‘0’ may beallowed. Therefore, a large-sized TIM is divided into small-sizedblocks/groups, STA can easily maintain TIM information, andblocks/groups may be easily managed according to class, QoS or usage ofthe STA. Although FIG. 13 exemplarily shows a 2-level layer, ahierarchical TIM structure comprised of two or more levels may beconfigured. For example, a total AID space may be divided into aplurality of page groups, each page group may be divided into aplurality of blocks, and each block may be divided into a plurality ofsub-blocks. In this case, according to the extended version of FIG.13(a), first N1 bits of AID bitmap may represent a page ID (i.e., PID),the next N2 bits may represent a block ID, the next N3 bits mayrepresent a sub-block ID, and the remaining bits may represent theposition of STA bits contained in a sub-block.

In the examples of the present invention, various schemes for dividingSTAs (or AIDs allocated to respective STAs) into predeterminedhierarchical group units, and managing the divided result may be appliedto the embodiments, however, the group-based AID allocation scheme isnot limited to the above examples.

Frame Structure

FIG. 14 is a diagram for explaining an exemplary frame format used in802.11 system.

A Physical Layer Convergence Protocol (PLCP) Packet Data Unit (PPDU)frame format may include a Short Training Field (STF), a Long TrainingField (LTF), a signal (SIG) field, and a data field. The most basic (forexample, non-HT) PPDU frame format may be comprised of a Legacy-STF(L-STF) field, a Legacy-LTF (L-LTF) field, an SIG field, and a datafield. In addition, the most basic PPDU frame format may further includeadditional fields (i.e., STF, LTF, and SIG fields) between the SIG fieldand the data field according to the PPDU frame format types (forexample, HT-mixed format PPDU, HT-greenfield format PPDU, a VHT PPDU,and the like).

STF is a signal for signal detection, Automatic Gain Control (AGC),diversity selection, precise time synchronization, etc. LTF is a signalfor channel estimation, frequency error estimation, etc. The sum of STFand LTF may be referred to as a PCLP preamble. The PLCP preamble may bereferred to as a signal for synchronization and channel estimation of anOFDM physical layer.

The SIG field may include a RATE field, a LENGTH field, etc. The RATEfield may include information regarding data modulation and coding rate.The LENGTH field may include information regarding the length of data.Furthermore, the SIG field may include a parity field, a SIG TAIL bit,etc.

The data field may include a service field, a PLCP Service Data Unit(PSDU), and a PPDU TAIL bit. If necessary, the data field may furtherinclude a padding bit. Some bits of the SERVICE field may be used tosynchronize a descrambler of the receiver. PSDU may correspond to a MACPDU defined in the MAC layer, and may include data generated/used in ahigher layer. A PPDU TAIL bit may allow the encoder to return to a stateof zero (0). The padding bit may be used to adjust the length of a datafield according to a predetermined unit.

MAC PDU may be defined according to various MAC frame formats, and thebasic MAC frame is composed of a MAC header, a frame body, and a FrameCheck Sequence. The MAC frame is composed of MAC PDUs, such that it canbe transmitted/received through PSDU of a data part of the PPDU frameformat.

A MAC header may include a frame control field, a Duration/ID field, anaddress field, etc. The frame control field may include controlinformation requisite for frame transmission/reception. The Duration/IDfield may be established as a specific time for transmitting thecorresponding frame or the like. For a detailed description of SequenceControl, QoS Control, and HT Control sub-fields of the MAC headerreference may be made to the IEEE 802.11-2012 standard documentation.

The frame control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame, and Order sub-fields. A detailed descriptionof individual sub-fields of the frame control field may refer to IEEE802.11-2012 standard documents.

The following [Table 1] shows a ‘To DS’ subfield and a ‘From DS’subfield contained in the frame control field defined in the legacy IEEE11 ac standard.

TABLE 1 To DS and From DS values Meaning To DS = 0, A data frame directsfrom one STA to another STA From DS = 0 within the same IBSS, a dataframe directs from one non-AP STA to another non-AP STA within the sameBSS, or a data frame escapes from the context of a BSS, as well as allmanagement and control frames. To DS = 1, A data frame destined for theDS or being sent by a From DS = 0 STA associated with an AP to the PortAccess Entity in that AP. To DS = 0, A data frame exiting the DS orbeing sent by the Port From DS = 1 Access Entity in an AP. To DS = 1, Adata frame using the four-address format. This From DS = 1 standard doesnot define procedures for using this combination of field values.)

Four address fields (Address 1, Address 2, Address 3, Address 4) of theMAC header may be used to indicate a Basic Service Set Identifier(BSSID), a Source Address (SA), a Destination Address (DA), aTransmitter Address (TA), a Receiver Address (RA), etc. Only some partsfrom among four address fields may be included according to frame type.The use of the address fields may be specified by relative positions ofthe address fields (Address 1-Address 4) of the MAC header, irrespectiveof address types of the corresponding field. For example, the receiveraddress (RA) may always be confirmed on the basis of contents of theAddress 1 field of the received frame. The receiver address (RA) of theCTS frame may always be obtained from the Address 2 field of thecorresponding RTS frame. The receiver address (RA) of the ACK frame mayalways be obtained from the Address 2 field of an objective frameindicating an ACK target. The following [Table 2] shows contents of theaddress fields (Address 1˜Address 4) of the MAC header according tovalues of ‘To DS subfield’ and ‘From DS subfield’ contained in the framecontrol field of the MAC header.

TABLE 2 To From Address 3 Address 4 DS DS Address 1 Address 2 MSDU caseA-MSDU case MSDU case A-MSDU case 0 0 RA = DA TA = SA BSSID BSSID N/AN/A 0 1 RA = DA TA = BSSID SA BSSID N/A N/A 1 0 RA = BSSID TA = SA DABSSID N/A N/A 1 1 RA TA DA BSSID SA BSSID

In [Table 2], RA is a receiver address, TA is a transmitter address, DAis a destination address, and SA is a source address. In addition, MSDUis a MAC Service Data Unit (SDU) serving as an information unitcommunicated between MAC Service Access Points (SAPs). A-MSDU(Aggregate-MSDU) is a format of a frame configured to transmit aplurality of MAC SDUs through one MAC PDU. The value of each addressfield (Address 1, Address 2, Address 3, or Address 4) may be set to anEthernet MAC address composed of 48 bits.

On the other hand, a Null-Data Packet (NDP) frame format may indicate aframe format having no data packet. That is, the NDP frame includes aPLCP header part (i.e., STF, LTF, and SIG fields) of a general PPDUformat, whereas it does not include the remaining parts (i.e., the datafield). The NDP frame may be referred to as a short frame format.

Duplicate Detection

MAC level acknowledgment (ACK) and retransmission is defined, such thatit may be possible to receive one frame one or more tunes. In this case,the duplicated frame should be filtered out. In order to filter out theduplicate frame, a sequence control field of the MAC header may be used.The Sequence Control field for use in the data frame and the managementframe is comprised of a Sequence Number and a Fragment Number. MPDUscorresponding to the same MSDU parts have the same sequence numbers, anddifferent MSDUs have different sequence numbers.

STA may allocate a sequence number of a frame according to a counter(for example, modulo-4096 counter starting from zero) increasing one byone per new MSDU. The STA for frame transmission is configured to store(or cache) the last sequence number for each Receiver Address (RA).

The STA for frame reception may cache the set of a Transmitter Address(TA), a sequence number, and a fragment number of the latest receptionframe. TA may be decided on the basis of the Address 2 field of thereceived frame. If the Retry field of the frame control field is set to1 and a frame having the same sequence number (or having the samefragment number) is received from the same TA, the reception STAdetermines a duplicated frame, and rejects the duplicated frame.

MAC Header Compression Method

The embodiment of the present invention proposes a compression method ofthe MAC header for low-power communication. For example, the MAC headercompression method proposed by the embodiment may use 1 MHz/2 MHz/4MHz/8 MHz/16 MHz channel bandwidths, and may be applied to a WLAN systemoperating in a frequency band of Sub 1 GHz (S1G).

Referring to FIG. 14, the MAC header may be necessarily included in aframe for data transmission. If the MAC header is reduced in size (i.e.,if overhead of the MAC header is reduced), generation, transmission,reception, etc. of the MAC frame of the STA may be more simplified,resulting in reduction of power consumption of the STA.

In addition, the WLAN system (for example, IEEE 802.11ah system)operating in Sub 1 GHz (S1G) is characterized in that it operates in alow frequency band and a coverage at which a frame arrives extends to 1km under an outdoor environment. The WLAN system is configured to mainlydefine a sensor- or meter-type STA having a low transfer rate and lowpower.

In addition, the power saving mechanism is of importance to thesensor-type STAs. For power saving, it is necessary for the sensor-typeSTAs to minimize the number of unnecessary awake situations, and thesensor-type STAs need to efficiently transmit transmission/receptiondata during an awake duration.

Accordingly, for the WLAN system operating in the S1G band, there is aneed to construct a frame for supporting long-range transmission and lowpower consumption. In order to implement a frame supporting long-rangetransmission, fields of the frame may be repeated at least twice on atime axis or a frequency axis at least twice. However, the size of theMAC header is increased in response to field repetition coding, suchthat power saving for frame processing of the STA may unavoidablyincrease.

In order to solve the above problem, the present invention provides aMAC header compression method. For this purpose, a method forconstructing a frame in a WLAN system operating in the S1G band willhereinafter be described in detail.

Communication for use in the S1G band has a larger coverage than thelegacy indoor WLAN system in terms of propagation characteristics, PHYdefined in the legacy IEEE 802.11ac system may be down-clocked to 1/10.In this case, each of 20/40/80/160/80+80 MHz channel bandwidthssupported by 802.11ac system is down-clocked to 1/10, such that2/4/8/16/8+8 MHz channel bandwidths may be provided to the S1G band.Accordingly, a Guard Interval (GI) may be increased from 0.8 μs to 8 μsin the 802.11ac system.

The legacy device is not present in the S1G band, such that a PHYpreamble optimum should be efficiently designed for the S1G band withoutconsidering backward compatibility. In accordance with the most simplemethod for solving the above requirement, the legacy HT-Green Field PLCPframe format (defined in IEEE 802.11n) is down-clocked to 1/10 so as todefine the S1G PHY preamble, and the above-mentioned structure mayexemplarily be applied to a bandwidth of 2 MHz or higher.

In order to support long-range communication, STF/LTF/SIG/DATA fields ofthe frame format of the S1G PHY structure for use in the bandwidth of 2MHz or higher are repeated twice or more times on a time axis or afrequency axis, such that along-range PLCP frame can be constructed.

FIG. 15 is a conceptual diagram illustrating an example of a long-rangePLCP frame format.

Although the PLCP frame format of FIG. 15 is comprised of STF, LIF1,SIG, LTF2-LTFN, and Data fields in a similar way to the Green-fieldformat defined in IEEE 802.11n, a transmission time of the preamble partmay increase two or more times by repetition as compared to theGreen-field. The PLCP frame format shown in FIG. 15 may be applied tothe 1 MHz bandwidth, and may be referred to as ‘1 MHz PPDU format’.

STF field of 1 MHz PPDU shown in FIG. 15 has the same periodicity asthat of an STF (having a length of two symbols) of a PPDU of thebandwidth of 2 MHz or higher, a twice-repetition (rep2) method isapplied to a time domain so that the STF field of the 1 MHz PPDU has thelength of 4 symbols (for example, mops) and the 3 dB power boosting isapplied thereto.

LTF1 field of the 1 MHz PPDU shown in FIG. 15 is orthogonal to anotherLTF1 field (having a length of 2 symbols) of a PPDU of the bandwidth of2 MHz or higher on a frequency domain, and is repeated twice on a timeaxis, such that the LTF1 field of the 1 MHz PPDU has a length of 4symbols.

S1G field of the 1 MHz PPDU shown in FIG. 15 may be repeatedly coded.Quadrature Phase Shift Keying (QPSK), Binary PSK (BPSK), etc. forModulation and Coding Scheme (MCS) may be applied to the S1G field of aPPDU of the bandwidth of 2 MHz or higher, and the S1G field has thelength of 2 symbols. In contrast, the lowest MCS (i.e., BPSK) and therepetition (rep2) coding is applied to the S1G field of the 1 MHz PPM,the S1G field of the 1 MHz PPDU has a rate of 1/2, and is defined tohave the length of 6 symbols.

Fields from the LTF2 field to the LTFN field of the 1 MHz PPDU shown inFIG. 15 may be applied to MIMO, and each LIT field may have a length ofone symbol.

The rep2 method may or may not be applied to the Data field of the 1 MHzPPDU shown in FIG. 15

FIG. 16 is a conceptual diagram illustrating a repetition (rep2) methodfor constructing a PLCP frame format of a 1 MHz bandwidth.

A scrambler shown in FIG. 16 may scramble data to reduce the probabilityof repeating ‘0’ or ‘1’ for a long time. Forward Error Correction (FEC)may encode data for error correction. For this purpose, the scramblermay include a binary convolution encoder or a Low Density Parity Check(LDPC) encoder.

In accordance with ‘2x block-wise repetition’, assuming that x encodedinformation bits of each OFDM symbol is repeated on a block basis tooutput 2x information bits. Here, assuming that the encoding rate isdenoted by 1/2, x/2 information bits of each OFDM symbol is encoded sothat x encoded information bits can be generated. After completion ofrepetition, assuming that the lowest MCS (for example, MCS0) is appliedto one Space Stream (SS), each symbol may include N_(CBPS) coded bits.

Thereafter, an interleaver may perform interleaving (or locationexchange) to prevent a contiguous noise bit from being repeated in along successive form. BPSK mapper may map the encoded data bit to theBPSK constellation point, or may map the encoded data bit to a complexsymbol. In the space mapping, time-space streams may be mapped totransmission chains. Through Inverse Discrete Fourier Transform (IDFT),complex symbols may be converted into a time-domain block. In GI &Window, some parts of a symbol are attached (or prepended) to the frontpart of the corresponding symbol so as to implement a guard interval(GI), edges of each symbol may be softened, and the windowing forincreasing spectral decay may be carried out. A transmission symbol maybe generated in analog and Radio Frequency (RF).

When the 1 MHz PPDU frame is constructed as described above, a durationof one PPDU is extremely lengthened, such that transmission efficiencymay be reduced and the STA power consumption may be increased. In orderto solve the above-mentioned problem, a method for reducing the lengthof a PPDU preamble and a method for compressing the MAC header may beused as necessary. The present invention provides a detailed MAC headercompression method capable of efficiently transmitting data in a WLANsystem.

The present invention assumes that the AP serves as a router. OpenSystem Interconnection (OSI) 7 layer obtained when a computer networkprotocol design and communication is divided into a plurality of layersis shown in the following [Table 3].

TABLE 3 Application Layer Presentation Layer Session Layer TransportLayer Network Layer Data Link Layer Physical Layer

Generally, if the AP does not operate as a router, the AP may operate asa physical layer and data link layers (i.e., MAC layer and Logical LinkControl (LLC) layer). Accordingly, there are needed four addresses(i.e., source address (SA), destination address (DA), transmitteraddress (TA), and a receiver address (RA)) in such a manner that the APreceives a frame and transmits the corresponding frame to a correctdestination. For this purpose, the header of the MAC frame for use inthe WLAN system may use four address fields as shown in FIG. 14.Contents of the four address fields may be determined according tovalues of ‘To DS subfield’ and ‘From DS subfield’ contained in the framecontrol field of the MAC header. Generally, the case in Which each ofthe ‘To DS’ field and the ‘From DS’ field is set to 1 is not present ina current WLAN system, such that the Address 4 field is not used.Accordingly, assuming that the AP does not operate as the router, threeaddress fields are needed in such a manner that the AP can receive theframe and transmit the corresponding frame to a correct destination.

On the other hand, assuming that the AP operates as a router, the AP mayperform various functions of a physical layer, a data link layer (i.e.,MAC layer, LLC layer, etc.), a network layer, a transport layer (forexample, a Transmission Control Protocol/Internet Protocol (TCP/IP)layer), etc. The AP may perform data transmission using only TA and RAother than SA and DA in the MAC layer. In this case, the IP layer mayperform correct frame transmission through SA and DA. In other words,assuming that the AP operates as a router, although only two addressfields indicating TA and RA (for example, AP address and STA address)are contained in the MAC header of the frame, such that correct frametransmission can be performed.

As described above, the AP must operate as a router to perform MACheader compression such that two address fields (TA and RA) arecontained as address information in the MAC header. However, each of APsdo not operate as a router, such that the AP must inform another STA ofcapability information indicating whether the AP can operate as therouter.

FIG. 17 is a conceptual diagram illustrating an example of an extendedcapability element according to an embodiment.

In FIG. 17, the Element ID field may be set to a specific valueindicating that the corresponding element is identical to the ExtendedCapabilities element. The Length field may be set to the number ofoctets corresponding to the length of Capabilities field. Capabilitiesfield may be a bit field indicating capability information of STA (or APSTA) configured to transmit the above element. The length ofCapabilities field may be denoted by a variable ‘n’, and the position ofeach bit may indicate whether specific capability is supported.

The present invention provides a method for adding, one bit indicatingwhether the MAC header compression function (i.e., indicating whetherthe router function is performed) is performed to the Capabilitiesfield. One bit may be a bit reserved in the Capabilities field. The STAhaving received the extended capability element from the AP confirms thevalue of one bit, and the AP operates as the router, such that the STAand the AP can recognize whether to perform MAC header compression. Theextended capability element may be contained in an associatedrequest/response frame, a re-associated request/response frame, a beaconframe, a probe response frame, etc.

As described above, assuming that MAC header compression is performed insuch a manner that the MAC header includes two address fields (TA andRA) acting as address information. TA and RA of the compressed MAC frameformat (also called a short MAC frame format) can be defined as shown inthe following [Table 4].

TABLE 4 Transmission direction Transmitter Address Receiver Address DLAP address STA address UL STA address AP address

As shown in [Table 4], TA and RA may be decided according totransmission direction. In the case of downlink (DL), TA is set to an APaddress, and RA is set to an address of the STA receiving a frame. Inthe case of uplink (UL), TA is set to an address of the STA configuredto transmit a frame, and RA is set to an address of the AP.

As described above, MAC header compression may be carried out in the MACheader such that address information is excluded from the MAC header(i.e., only requisite RA and TA are contained and other addressinformation is omitted). In addition, the present invention provides amethod for reducing overhead of address information contained in the MACheader.

As described above, the address field of the legacy MAC header isconfigured to have a MAC address of 48 bits. However, the presentinvention provides a method for using an associated identifier (AID)instead of the MAC address of the STA so as to compress addressinformation. AID is defined to have the length of 16 bits. Accordingly,overhead of the MAC header can be greatly reduced when AID is used. TAand RA of the compressed MAC header proposed by the present inventionmay be defined as shown in the following [Table 5].

TABLE 5 Transmission direction Transmitter Address Receiver Address DLBSSID STA AID UL STA AID BSSID

As shown in [Table 5], in the case of downlink (DL), TA (for example,Address 2 field) is set to BSSID, and RA (for example, Address 1 field)is set to an AID of the STA having received the frame. In the case ofUplink (UL), TA (for example, Address 2 field) is set to an AID of theSTA having transmitted a frame, and RA (for example, Address 1 field) isset to BSSID. BSSID may be identical to MAC address of the AP.

Method for Detecting Repetition of Frame Including Compressed MAC Header

If the MAC address of the STA is replaced with an AID in the MAC header,the STA having received the frame changes (or maps) the AID contained inthe MAC header of the frame to the MAC address, and the STA stores thechanged (or mapped) MAC address in a memory (or cache memory) along witha sequence number. As a result, retransmission of the compressed MACframe can be supported.

For example, the STA having received a DL frame from the AP stores notonly the MAC address corresponding to a BSSID contained in the TAaddress field (i.e., Address 2 field) of the DL frame, but also thesequence number in the cache memory. If access category information iscontained in the DL frame, BSSID, Sequence Number, and Access Categoryare stored in the cache memory.

The AP having received a UL frame from the STA may confirm the STA AIDcontained in the TA address field (i.e., Address 2 field) of the ULframe. Since the STA AID is allocated by the AP, the AP recognizes theMAC address (i.e., the mapping relationship between STA AID and STA MACaddresses) to which the corresponding AID is allocated. Accordingly, theAP may recognize the STA MAC address on the basis of the STA AIDcontained in the address field (i.e., Address 2 field) of the UL frame.The AP may store not only the STA MAC address (mapped to AID) identifiedby AID, but also Sequence Number in the cache memory. If Access Categoryinformation is contained in the UL frame, STA MAC Address, SequenceNumber, and Access Category are stored in the cache memory.

The STA may manage the cache according to the sequence control schemeproposed by the present invention, such that correct retransmission ofthe compressed MAC frame (or short MAC frame) can be carried out.Specifically, in order to perform correct retransmission under theenvironment in which a frame including a normal MAC header and a frameincluding a compressed MAC header are used, the MAC header compressionscheme and the sequence control scheme proposed by the present inventionare needed.

For example, after a first STA transmits a first frame in which thecompressed MAC header is used to a second STA, a normal MAC header maybe used in a second frame transmitted to the second STA. Here, the firstframe and a second frame are configured to transmit different MPDUs. Inthis case, since each of the compressed MAC frame and the normal MACframe is retransmitted, a unified cache maintenance scheme is needed toefficiently determine the presence or absence of duplicated reception.Otherwise, a cache managed on the basis of an AID and a sequence numberand a cache managed on the basis of a MAC address and a sequence numbermust be maintained not only in the frame transmission STA but also inthe frame reception STA, resulting in increased costs of the STA. Inaddition, assuming that different MPDUs corresponding to parts of oneMDSU are transmitted through a frame of a normal MAC header or a frameof a compressed MAC header, sequence control information must be managedusing the same sequence number and different fragment numbers within aspecific STA. Assuming that a sequence number based on the AID and asequence number based on the MAC address are managed independently fromeach other, although repetition of such frames is detected, there mayoccur a malfunction in which the repeated frames cannot be correctlyprocessed.

Accordingly, in association with the frame contained in the compressedMAC header configured to use the STA AID, the present invention providesa method for storing not only the STA MAC address (or mapped to STA AID)identified by the STA AID but also a sequence number in the cachememory.

In the frame transmission STA, a sequence number of the transmissionframe is sequentially increased per RA or per {RA, access category}. Inaccordance with the proposal of the present invention, assuming that theRA address field (i.e., Address 1 field) of the transmission frame is acompressed MAC frame configured in the form of STA AID, the sequencenumber of the transmission STA is managed on the basis of the MACaddress of the receiver STA, instead of on the basis of the AID of thereceiver STA. That is, the STA having transmitted the frame may store(or cache) the last sequence number per MAC address of the receiver STA.

A retry bit of the frame control field of the retransmitted frame is setto 1. Assuming that a frame having the retry bit of 1 is received andthe received frame uses the compressed MAC header, STA AID contained inthe address field of the compressed MAC header is converted into the STAMAC address. STA having received the frame may compare the converted STAMAC address (or MAC address identified by the STA AID value contained inthe address field of the received frame), a sequence number, and/oraccess category information with past cache information (i.e., the laststored STA MAC address, a sequence number, and access categoryinformation), such that the STA may determine whether a currentreception frame is a duplicated frame.

Encryption of Short MAC Header

The present invention proposes a method for encrypting a short MAC frame(or a compressed MAC frame).

An encryption method of a frame configured to use a normal MAC headermay be different from an encryption method of a frame configured to usea short MAC header. As shown in the following description, a method forconstructing Additional Authentication Data (AAD) and a method forconstructing a Nonce for use in a first case in which a normal MACheader is used are different from those of the other case in which ashort MAC header is used. Accordingly, in order to correctly performintegrity verification of the MAC header, the present invention proposesa method for applying the same frame format to transmission andretransmission of the same MPDU.

For example, after the MPDU is transmitted using a normal MAC frame (ornormal MAC header), a short MAC frame (or MAC header) cannot be used inretransmission of the same MPDU, and the same MPDU may be retransmittedusing a normal MAC frame (or normal MAC header). In addition, after theMPDU is transmitted using a short MAC frame 9 or a short MAC header), anormal MAC frame (or a normal MAC header) cannot be used inretransmission of the same MPDU, and the same MPDU may be retransmittedusing a short MAC frame (or a short MAC header).

FIG. 18 is a block diagram illustrating CCMP (Counter mode withCipher-block chaining Message authentication code Protocol)encapsulation.

For encryption of the MAC frame in IEEE 802.11, Temporal Key IntegrityProtocol (TKIP), Counter mode with Cipher-block chaining Messageauthentication code Protocol (CCMP), etc. may be used. CCMP was proposedby IEEE 802.11i standard. CCMP is an enhanced cryptographicencapsulation method designed for confidentiality on the basis of CCM ofAdvanced Encryption Standard (AES).

A security mechanism for use in IEEE 802.11 may be provided to a dataframe and a management frame. In more detail, data confidentiality,authentication, integrity, replay protection, etc. may be provided usingTKIP, CCMP, etc.

Referring to the example of FIG. 18, it may be possible to obtain anencrypted MPDU from payload of a plaintext MPDU.

In more detail, a Packet Number (PN) may be increased to obtain a new PNvalue of each MPDU.

AAD for CCM may be constructed using fields of the MAC header of theplaintext MPDU. The CCM algorithm may provide integrity protection offields contained in the AAD. AAD may include a Frame Control (FC) field,an A1 (Address 1) field, an A2 (Address 2) field, an A3 (Address 3)field, a SC (Sequence Control) field, an A4 (Address 4) field, and a QC(QoS Control) field.

CCM Nonce may be constructed on the basis of a PN value, an A2 (Address2) field of MPDU, and a priority value. Nonce may represent a number ora bit string used only once in the security algorithm.

8-octet CCMP header may be formed on the basis of a PN value and a keyidentifier (KeyID).

Encrypted data and MIC (Message Integrity Code) may be formed usingTemporal key (TK), AAD, Nonce, and MPDU data.

The original MPDU header, the generated CCMP header, the generatedencrypted data, and MIC are combined with one another, such that theencrypted MPDU is formed.

FIG. 19 is a conceptual diagram illustrating a Frame Control (FC) fieldof a short MAC header according to an embodiment.

The subfields of the FC field in the short MAC header illustrated inFIG. 19 may be configured partially differently from the subfields of anormal MAC header described before with reference to FIG. 14.

For example, while the 2-bit Type field of the normal MAC header is 2bits long, the Type field of the FC field of the short MAC header is 3bits long. Also, while the Subtype field of the normal MAC header is 4bits long, the Subtype field of the FC field of the short MAC header is3 bits long. Unlike the normal MAC header, the FC field of the short MACheader does not include a To DS field, a Retry field, and an Orderfield. On the other hand, the FC field of the short MAC header includesan End Of Service Period (EOSP) field, a Relayed Frame field, and an AckPolicy field, which is different from the normal MAC header.

As noted from the exemplary format of a short MAC header illustrated inFIG. 19, the FC field of the short MAC header according to the presentinvention characteristically includes a Protocol Version field (2 bits),a Type field (3 bits), a Subtype field (3 bits), a From DS field (1bit), a More Fragments field (1 bit), a Power Management field (1 bit),a More Data field (1 bit), a Protected Frame field (1 bit), an EOSPfield (1 bit), a Relayed Frame field (1 bit), and an Ack Policy field (1bit).

In addition, as described before with reference to FIG. 18 AAD isconfigured with fields of a MAC header, and a method for configuring AADin the case where the FC field of a short MAC header as illustrated inFIG. 19 is used will be described below with reference to FIG. 20.

FIG. 20 illustrates an exemplary structure of AAD according to thepresent invention.

In the example of FIG. 20, FC represents an FC field, which may have 2octets in size.

In FIG. 20, the FC field of the AAD may be configured according to theFC field of the short MAC header illustrated FIG. 19. The Type bits ofthe FC field may be masked to 0 in the AAD. The Power Management bit ofthe FC field may also be masked to 0 in the AAD. Additionally, the MoreData bit of the FC field may be masked to 0 in the AAD. The ProtectedFrame bit of the FC field may always be set to 1 in the AAD. Also, theEOSP bit of the FC field may be masked to 0 in the AAD. The RelayedFrame bit of the FC field may also be masked to 0 in the AAD. The AckPolicy bit of the FC field may also be masked to 0 in the AAD. Masking afield to a value of 0 may be understood as inclusion of the field in theAAD but non-use of the field.

In FIG. 20, A1, A2, A3, and A4 fields correspond to Address 1, Address2, Address 3, and Address 4 fields of an MPDU, respectively. The A1field may be 6 or 2 octets long, the A2 field may be 6 or 2 octets long,the A3 field may be 6 or 0 octets long (if the A3 field has a zerooctet, this means that the A3 field may be omitted), and the A4 fieldmay be 6 or 0 octets long (if the A4 field has a zero octet, this meansthat the A4 field may be omitted).

Specifically, the short MAC header may be configured by omitting one ormore of the A3 and A4 fields and always including the A1 (i.e., RA) andA2 (i.e., TA) fields, as described before in relation to [Table 4] and[Table 5]. If the A1 field is configured as a MAC address or a BSSID,the A1 field may have 6 octets, and if the A1 field is configured as anAID, the A1 field may have 2 octets. Also, if the A2 field is configuredas a MAC address or a BSSID, the A2 field may have 6 octets, and if theA2 field is configured as an AID, the A2 field may have 2 octets.

In this manner, one or both of the A3 and A4 fields may be omitted inthe AAD. For example, if the A3 field is omitted in the short MACheader, the AAD may be configured with the FC, A1, A2, A4, and SCfields. Or if the short MAC header is free of the A4 field, the AAD maybe configured with the FC, A1, A2, A3, and SC fields. Or if the shortMAC header is free of the A3 and A4 fields, the AAD may be configuredwith the FC, A1, A2, and SC fields.

Herein, the A1 field of the AAD may have 6 or 2 octets in size.

Specifically, the A1 field of the AAD illustrated in FIG. 20 isconfigured according to the Address 1 field of an MPDU. The A1 field ofthe AAD may be configured as an AID (2 octets) or a MAC address (6octets according to a frame direction (e.g., a UL frame or a DL frame).In the case of a DL frame with the From DS bit of the FC field set to 1in the short MAC header (in this case, the From DS bit of the FC fieldis also set to 1 in the AAD), the A1 field of the AAD is configured tothe AID (2 octets) of a receiving STA. Or in a UL frame with the From DSbit of the FC field set to 0 in the short MAC header (in this case, theFrom DS bit of the FC field is also set to 1 in the AAD), the A1 fieldof the AAD is configured as the MAC address or BSSID (6 octets) of areceiving STA (or an AP).

Further, the A2 field of the AAD may be 6 or 2 octets long.

Specifically, the A2 field of the AAD illustrated in FIG. 20 isconfigured according to the Address 2 field of an MPDU. The A2 field ofthe AAD may be configured as an AID (2 octets) or a MAC address (6octets) according to a frame direction (e.g., a UL frame or a DL frame).In the case of a DL frame with the From DS bit of the FC field set to 1in the short MAC header (in this case, the From DS bit of the FC fieldis also set to 1 in the AAD), the A2 field of the AAD is configured asthe MAC address or BSSID (6 octets) of a transmitting STA (or an AP). Orin a UL frame with the From DS bit of the FC field set to 0 in the shortMAC header (in this case, the From DS bit of the FC field is also set to0 in the AAD), the A2 field of the AAD is configured as the AID (2octets) of a transmitting STA.

If present, the A3 field illustrated in FIG. 20 is configured accordingto the Address 3 field of an MPDU. Also, if present, the A4 fieldillustrated in FIG. 20 is configured according to the Address 4 field ofan MPDU.

In FIG. 20, SC represents an SC field, which may have 2 octets. The SCfield of the AAD illustrated in FIG. 20 may be configured according tothe SC field of an MPDU.

As described before in the foregoing Duplicate Detection section, the SCfield of a MAC header includes Sequence Number and Fragment Numbersubfields, and the SC field of the AAD illustrated in FIG. 20 alsoincludes Sequence Number and Fragment Number subfields. The SequenceNumber subfield (bit 4 to bit 15) of the SC field may be masked to 0 inthe AAD illustrated in FIG. 20. Further, compared to the Fragment Numbersubfield of the SC field in the MAC header, the Fragment Number subfieldof the SC field is not modified in the AAD illustrated in FIG. 20.

It is to be understood that the sequence of the AAD componentsillustrated in FIG. 20 is not limited and an AAD configured according tothe present invention characteristically includes a part of thesubfields illustrated in the example of FIG. 20.

FIG. 21 illustrates an exemplary configuration of Nonce according to thepresent invention.

As illustrated in FIG. 21, the Nonce may include a Nonce Flags field, aSTA MAC address identified by A2 (Address 2) field, and a PN field. TheNonce Flags field may have 1 octet. The STA MAC address identified by A2(Address 2) field may have 6 octets. The PN field may have 6 octets.

In FIG. 21, the specific structure of the Nonce Flags field isadditionally illustrated. The Nonce Flags field may include a 4-bitPriority subfield, a 1-bit Management subfield, and 3 reserved bits.

The Priority subfield of the Nonce Flags field illustrated in FIG. 21may be set to a value indicating the priority of a short MAC frame. Forexample, the Priority subfield may be set to a value indicating theTraffic Identifier (TID) of a plaintext MPDU or a value indicating anAccess Category (AC).

The Management subfield of the Nonce Flags field illustrated in FIG. 21may be set to a value indicating whether a plaintext MPDU is amanagement frame.

The A2 field of the Nonce illustrated in FIG. 21 may be configuredaccording to the Address 2 field of a short MAC header. The A2 field ofthe Nonce may be configured as the AID (2 octets) or MAC address (6octets) of a transmitting STA. In the case of a DL frame having the FromDS bit of the FC field set to 1 in a short MAC header, the A2 field ofthe Nonce may be configured as the MAC address or BSSID (6 octets) of atransmitting STA (or AP). For example, the A2 field of the Nonce may beconfigures as the MAC address or BSSID (6 octets) of a transmitting STA(or AP) identified by the A2 field of the short MAC header. Or in thecase of a UL frame having the From DS bit of the FC field set to 0 in ashort MAC header, the A2 field of the Nonce may be configured as the AID(2 octets) of a transmitting STA.

The STA MAC Address identified by A2 field of the Nonce illustrated inFIG. 21 may be configured according to the Address 2 field of a shortMAC header and determined according to a frame direction (e.g., a ULframe or a DL frame). Specifically, the STA MAC Address identified by A2field may be set to the STA MAC address of a transmitting STA identifiedby an AID (2 octets) in a UL frame, and the BSSID included in the A2field in a DL frame.

FIG. 22 illustrates an exemplary structure of an encrypted MPDU.

As described before with reference to FIG. 18, an encrypted MPDUresulting from encrypting a plaintext MPDU may include a MAC header asillustrated in FIG. 22 (the MAC header of a plaintext MPDU asillustrated in FIG. 18), a CCMP header as illustrated in FIG. 22 (a CCMPheader generated based on a PN and KeyId as illustrated in FIG. 18),generated encrypted data as illustrated in FIG. 22, an MIC, and a FrameCheck Sequence (FCS).

In CCMP, it is required that a temporal key is updated in each session,and additionally, a nonce value is unique for a given temporal key inevery frame. To satisfy these requirements, a 48-bit PN value is used,and the PN value is initialized to 1 each time a temporal key isupdated.

In the example of FIG. 22, a PN value may be transmitted in the CCMPheader. The CCMP header includes a 6-octet (i.e., 48-bit) PN field, andthe 6 octets are called PN0, PN1, PN2, PN3, PN4, and PN5, respectively.

In the present invention, it is proposed that the MAC overhead of anencrypted PPDU is further reduced by decreasing the size of the PN fieldin a short MAC frame.

Specifically, only a part (e.g., PN0 and PN1) of the 6 octets of the PNmay be transmitted in the CCMP healer, whereas the other octets (e.g.,PN2, PN3, PN4, and PN5) may be synchronized between a transmitting STAof the MAC frame and a receiving STA of the MAC frame.

For example, when a STA initially transmits an encrypted PPDU, the STAmay transmit an entire 48-bit PN value in the normal MAC frame formatinstead of the short MAC frame format.

If both of the transmitting and receiving STAs support the short MACframe, the receiving STA may store or keep the 48-bit PN value of theencrypted PPDU transmitted in the normal MAC frame format. For example,for a PPDU which has been successfully received, decrypted, and thusverified for integrity, a cash for a set of {Transmitter Address,Temporal Key, PN 48 bits} may be stored, kept, and managed in thereceiving STA.

In this manner, after a PN value is synchronized between transmittingand receiving STAs, the transmitting STA may transmit a PPDU resultingfrom encrypting a short MAC frame (different from an encrypted PPDUtransmitted previously in a normal MAC frame). Subsequently, a CCMPheader included in a short MAC frame may include only a part (e.g., PN0and PN1) of the 48-bit PN value, thereby reducing MAC overhead.

Upon receipt of the PPDU resulting from encrypting the short MAC frame,the receiving STA may use the previously stored PN value in decryptingthe short MAC frame. That is, if only PN0 and PN1 are included in theCCMP header of the short MAC frame, the receiving STA may configure theentire 48-bit PN value using the stored values as PN2, PN3, PN4, andPN5. In this manner, the receiving STA may decrypt the MAC frame usingthe 48-bit PN value configured by combining the part included in theCCMP header with the stored other part (that is, determining that thecombined PN value has been used in Nonce configuration).

If a temporal key is changed, the receiving STA deletes the PN valuestored as a set of {Transmitter Address, Temporal Key, PN 48 bits}.Accordingly, if the temporal key is changed, the transmitting STA shouldtransmit an entire 48-bit PN value in the normal MAC frame format,instead of the short MAC frame format. Thus, the PN value may besynchronized between the transmitting and receiving STAs.

Meanwhile, as described before in the foregoing Duplicate Detectionsection, a MAC header includes an SC field, and the value of theSequence Number subfield of the Sequence Control field is increased by 1in each PPDU. The present invention proposes that MAC overhead isfurther reduced by using the value of Sequence Number as a part of a PNvalue (or by associating the value of Sequence Number with a part of aPN value).

In this case, an entire PN value may be indicated to the receiving STAin an initial transmitted frame. The receiving STA may store a set ofvalues including the value of Sequence Number in the Sequence Controlfield of the MAC header of a current received frame, along with theentire PN value. For example, the receiving STA may store, keep, andmanage a set of {Transmitter Address, Temporal Key, PN 48 bits, SequenceNumber} in a cache. In subsequent transmissions, a PN field may not beincluded in a CCMP header. In this case, the receiving STA may determinea PN value using the value of Sequence Number included in the SequenceControl field of an encrypted MPDU generated out of a short MAC frame.

Also, if the temporal key is changed, the receiving STA deletes the PNvalue which has been stored as the set of {Transmitter Address, TemporalKey, PN 48 bits, Sequence Number}. Accordingly, if the temporal key ischanged, the transmitting STA should transmit a whole 48-bit PN value inthe normal MAC frame format instead of the short MAC frame format. Thus,the PN value may be synchronized again between the transmitting andreceiving STAs.

If the Sequence Number is used as a part of the PN value, the SequenceNumber may also be initialized along with the initialization of the PNvalue due to the change of the temporal key.

A part of the PN value, for example, PN0∥PN1 (∥ represents concatenationof PN0 and PN1) corresponding to the Sequence Number may correspond tothe value of the SC field. In this case, the PN value may be calculated(or recovered) using PN0∥PN1 corresponding to the SC field and PN2 toPN5 stored in the receiving STA by [Equation 1].

$\begin{matrix}\begin{matrix}{{PN} = {{SC}{{{PN}\; 2{{{PN}\; 3{{{PN}\; 4{{{PN}\; 5}}}}}}}}}} \\{= {{PN}\; 0{{{PN}\; 1{{{PN}\; 2{{{PN}\; 3{{{PN}\; 4{{{PN}\; 5}}}}}}}}}}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In [Equation 1], PN2∥PN3∥PN4∥PN5 may be referred to as a Base PN (BPN).Thus, it may be expressed that PN=SC∥BPN.

If the Sequence Number value of a current received frame is smaller thanthe Sequence Number value of a previously received frame, a STAincreases a stored BPN (i.e., PN2∥PN3∥PN4∥PN5) by 1. This may beunderstood as increasing the highest digit of the PN0∥PN1 value of theSC field by 1 since if the PN0∥PN1 value is sequentially increased toabove a maximum value, it becomes a minimum value cyclically (this maybe called roll-over) and then is increased by 1.

FIG. 23 is a view illustrating an MSDU reception flow in a MAC dataplane architecture.

As illustrated in FIG. 23, upon receipt of an MPDU, if the received MPDUis an A-MPDU, a STA may de-aggregate the A-MPDU into individual MPDUs.

Further, the STA may validate whether an MPDU header and a CRC arevalid, for each MPDU.

If a frame is valid without an error, the STA may perform filtering todetermine whether the received frame is destined for the STA based onAddress 1 (i.e., RA) included in the MAC header of the frame.

If the STA confirms that the frame is destined for the STA, the STAdetermines whether the frame is a duplicate of a previously receivedframe based on the TA, Sequence Number, and Fragment Number of theframe, and performs duplicate removal.

If the frame is not a duplicate, the STA performs MPDU decryption andintegrity check optionally, when needed.

After the decryption and integrity check, the STA performs Block ACKreordering. Block ACK reordering means an operation in which thereceiving STA buffers and manages a plurality of successfully receivedMPDUs until the MPDUs are completely ordered in their originaltransmission order in consideration of MPDUs to be retransmitted, asindicated by a Block ACK, without immediately transmitting the MPDUs toa higher layer or a higher-layer MAC entity. Block ACK reordering of aplurality of frames may include, for example, ordering the frames in anascending order of Sequence Number values and discarding a framecorresponding to a Sequence Number already existing in an ACK buffer.

Subsequent de-fragmentation is an operation of recovering originalinformation by combining a plurality of fragments.

Then, an MSDU reception process may be performed through integrity checkand reporting for an MSDU (optional), replay detection (for a non-meshSTA), received MSDU rate limiting, and the like.

If the value of the SC field of a short MAC frame is a part (e.g.,PN0∥PN1) of a PN value according to the present invention, as a SequenceNumber rolls over, a BPN (i.e., PN2∥PN3∥PN4∥PN5) kept/stored in thereceiving STA is increased by 1.

However, in the case where a short MAC frame is subjected to MPDUdecryption and integrity check after duplicate removal as illustrated inFIG. 23, if the transmitting STA successively transmits a plurality ofMPDUs, the receiving STA may face a problem with PN update.

For example, the transmitting STA may configure one A-MPDU byaggregating a plurality of MPDUs and transmit the A-MPDU as a singlePPDU. The STA receiving this PPDU configures ACK information for eachindividual MPDU of the A-MPDU and feeds back the ACK information to thetransmitting STA in a control frame called a Block ACK frame. Uponreceipt of the Block ACK frame, the transmitting STA retransmits an MPDUindicated as erroneous by the Block ACK frame.

If a plurality of short MAC frames are combined to an A-MPDU and thentransmitted, it is assumed that the Sequence Numbers of the combinedindividual short MAC frames are N−2, N−1, N, 0, 1, and 2, respectively.It is also assumed that the short MAC frames with the Sequence Numbersof N and 0 have errors, and the short MAC frames with the SequenceNumbers of N−2, N−1, 1, and 2 are successfully received without errors.

In this case, the receiving STA performs MPDU decryption and integritycheck before Block ACK reordering for the received short MAC frames(i.e., irrespective of the actual transmission order of the receivedframes). Then, during processing the short MAC frame having the SequenceNumber of 1, the STA determines that the frame has a Sequence Numbersmaller than a previously received frame and increases its stored BPN(i.e., PN2∥PN3∥PN4∥PN5) by 1. That is, since the receiving STA hasfailed to receive the frames having the Sequence Numbers of N and 0, itreceives the frame having the Sequence Number of 1 smaller than theSequence Number of a previously received frame being N−1, and thusincreases the BPN by 1. Because the receiving STA has increased the BPNwithout successfully receiving the frames having the Sequence Numbers ofN and 0 yet, if the receiving STA successfully receives the frameshaving the Sequence Numbers of N and 0 through retransmission, thereceiving STA does not perform MPDU decryption and integrity checknormally for these short MAC frames.

Therefore, the present invention proposes that in the case where the SCfield of a short MAC frame is a part (e.g., PN0∥PN1) of a PN, only whenMPDU decryption and integrity check are performed after received shortMAC frames are sequentially ordered in an actual transmission orderthrough Block ACK reordering in consideration of successive transmissionof a plurality of short MAC frames, the receiving STA increases a storedBPN (e.g., PN2∥PN3∥PN4∥PN5) by 1 in the case of Sequence Numberroll-over (that is, upon receipt of a frame having a Sequence Numbersmaller than a previously received Sequence Number).

In other words, it may be defined that only when a Block ACK is not usedor decryption is performed after Block ACK reordering, if the SequenceNumber of a received MPDU is smaller than the Sequence Number of apreviously received MPDU, a BPN (e.g., PN2∥PN3∥PN4∥PN5) stored in areceiving STA is increased by 1.

Meanwhile, if it is defined that MPDU decryption and integrity check areperformed after Block ACK reordering, a Block ACK reordering buffer maybe updated wrongly for an MPDU that has not passed the integrity check(i.e., an MPDU experiencing integrity check failure). That is, since thereceiving STA has no way to determine whether an MPDU will pass theintegrity check during Block ACK reordering, the receiving STA shouldstore all MPDUs as already received in the Block ACK reordering buffer.Then, if an MPDU with integrity check failure is retransmitted, theBlock ACK reordering buffer may discard the MPDU, considering that theMPDU is a duplicate of a previously received MPDU (i.e., an MPDUreceived successfully from the transmitting STA).

To avert this problem, MPDU decryption and integrity check shouldprecede Block ACK reordering, and in the case of successivetransmissions of a plurality of STAs, the transmitting STA shouldtransmit the MPDUs in such a manner that there may be no MPDUs for whichreception of an ACK is awaited before roll-over of the Sequence Numbersof the MPDUs.

Specifically, if a plurality of short MAC frames are combined into anA-MPDU and transmitted, N−2, N−1, N, 0, 1, and 2 should not be allowedas the Sequence Numbers of the short MAC frames. This means theconstraint that before roll-over of the Sequence Number from N to 0,ACKs should be received for MPDUs having Sequence Numbers of N−2 andN−1.

Therefore, the transmitting STA may combine only the short MAC frameshaving the Sequence Numbers of N−2, N−1, and N prior to transmission,and transmit the short MAC frame with the Sequence Number of 0 only inthe absence of another MPDU for which reception of an ACK is awaited inthe transmitting STA.

FIG. 24 is a flowchart illustrating another exemplary method accordingto the present invention.

In step S2410, a STA may receive a frame (e.g., an MPDU).

In step S2420, the STA may determine a PN value using the value of theSC field of the received frame and a partial PN (or BPN) stored in theSTA.

In step S2430, the STA may perform decryption for the frame using the PNvalue.

If the decryption is performed after Block ACK reordering, the STA mayperform an operation of increasing the partial PN value (or BPN value)stored in the STA by 1 due to roll-over of the value of the SC field. Ifthe decryption is performed before the Block ACK reordering, theoperation for increasing the partial PN value (or BPN value) stored inthe STA by 1 due to roll-over of the value of the SC field should not beperformed.

If a Bloc ACK itself is not used, the operation for increasing thepartial PN value (or BPN value) stored in the STA by 1 due to roll-overof the value of the SC field may be performed irrespective of thesequence of the decryption and the Block ACK reordering.

While the exemplary method depicted in FIG. 24 is described as a seriesof operations, for clarity of description, this does not limit the orderof steps. When needed, the steps may be performed at the same time or ina different order. Moreover, all steps depicted in FIG. 24 are notneeded to implement the method proposed by the present invention.

The method depicted in FIG. 24 may be implemented so that the foregoingvarious embodiments of the present invention may be appliedindependently or two or more of them may be applied simultaneously.

FIG. 25 is a block diagram of a wireless apparatus according to anembodiment of the present invention.

A STA 10 may include a processor 11, a memory 12, and a transceiver 13.The transceiver 13 may be configured to transmit and receive wirelesssignals, and implement the PHY layer according to, for example, an IEEE802 system. The processor 11 may be connected to the transceiver 13 andimplement the PHY layer and/or the MAC layer according to the IEEE 802system. The processor 11 may be configured to perform operationsaccording to the foregoing various embodiments of the present invention.Also, modules for performing STA operations according to the variousembodiments of the present invention may be stored in the memory 12 andexecuted by the processor 11. The memory 12 may reside inside or outsideof the processor 11 and may be connected to the processor 11 by a knownmeans. The STA 10 illustrated in FIG. 25 may be an AP STA or a non-APSTA.

The processor 11 of the STA 10 illustrated in FIG. 25 may be configuredto receive a frame by controlling the transceiver 13. In this case, theprocessor 11 may be configured to determine a PN value using the valueof the SC field of a received frame and a partial PN value (or BPNvalue) stored in the memory 12. Further, the processor 11 may beconfigured to decrypt the received frame using the determined PN value.If a Block ACK is used for a plurality of frames including the receivedframe, only when the decryption is performed after Block ACK reordering,the processor 11 is allowed to perform an operation of increasing thepartial PN value (i.e., BPN value) stored in the STA by 1 due toroll-over of the value of the SC field.

The STA 10 illustrated in FIG. 25 may be configured specifically in sucha manner that the descriptions of the foregoing various embodiments maybe implemented independently or two or more of the embodiments may beimplemented simultaneously, and a redundant description is avoided forclarity.

The Embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

While the various embodiments of the present invention have beendescribed above in the context of an IEEE 802.11 system, the same thingis applicable to many other mobile communication systems.

The invention claimed is:
 1. A method for receiving a frame at a Station(STA) in a wireless communication system, the method comprising:receiving a frame including a Sequence Control (SC) field, the framebeing encrypted using a packet number (PN) and the SC field including asequence number for the frame; determining a first portion of the PNusing the SC field and a second portion of the PN using a partial PNvalue stored in the STA; and performing decryption for the frame usingthe PN, in determining the second portion of the PN, when the decryptionfor the frame is performed after block Acknowledgment (ACK) reordering,and the sequence number for the frame is smaller than a previoussequence number, the partial PN value stored in the STA is increasedby
 1. 2. The method according to claim 1, wherein if block ACK is notused for the frame and the sequence number of the SC field of thereceived frame is smaller than the previous sequence number, the partialPN value stored in the STA is increased by
 1. 3. The method according toclaim 1, wherein the block ACK reordering includes ordering a pluralityof frames including the frame in an ascending order of sequence numbers.4. The method according to claim 1, wherein the PN is 48 bits length andthe PN is determined by concatenating PN0, PN1, PN2, PN3, PN4, and PN5,each being 8 bits length.
 5. The method according to claim 4, whereinthe SC field is set to a value obtained by concatenating PN0 and PN1. 6.The method according to claim 4, wherein the first portion of the PNincludes PN0 and PN1, and the second portion of the PN includes PN2,PN3, PN4, and PN5.
 7. The method according to claim 1, wherein when thesequence number is rolled over, the sequence number of the SC field issmaller than the previous sequence number.
 8. The method according toclaim 1, wherein the frame is a MAC Protocol Data Unit (MPDU).
 9. AStation (STA) for receiving a frame in a wireless communication system,the STA comprising: a transceiver to receive a frame including aSequence Control (SC) field, the frame being encrypted based on a packetnumber (PN) and the SC field including a sequence number for the frame;and a processor to determine a first portion of the PN using the SCfield, to determine a second portion of the PN using a partial PN valuestored in the STA, and to perform decryption for the frame using the PN,wherein in determining the second portion of the PN, when the decryptionfor the frame is performed after block Acknowledgment (ACK) reordering,and the sequence number for the frame is smaller than a previoussequence number, the partial PN value stored in the STA is increasedby
 1. 10. The STA according to claim 9, wherein if block ACK is not usedfor the frame and the sequence number of the SC field of the receivedframe is smaller than the previous sequence number, the partial PN valuestored in the STA is increased by
 1. 11. The STA according to claim 9,wherein the block ACK reordering includes ordering a plurality of framesincluding the frame in an ascending order of sequence numbers.
 12. TheSTA according to claim 9, wherein the PN is 48 bits length and the PN isdetermined by concatenating PN0, PN1, PN2, PN3, PN4, and PN5, each being8 bits in length, and wherein the first portion of the PN includes PN0and PN1, and the second portion of the PN includes PN2, PN3, PN4, andPN5.
 13. The STA according to claim 12, wherein the SC field is set to avalue obtained by concatenating PN0 and PN1.
 14. The STA according toclaim 9, wherein when the sequence number is rolled over, the sequencenumber of the SC field is smaller than the previous sequence number. 15.The STA according to claim 9, wherein the frame is a MAC Protocol DataUnit (MPDU).